So far, AT&T has been enjoying exclusivity of the HTC One X in the US (theirs is a renamed Krait-powered One XL), but now a rumor based on a leaked shot of T-Mobile's inventory shows that the carrier will be getting the "HTC One X+".
So, what's the difference between One X and One X+? Apparently, the plus version will use a Tegra 3+ chipset. The updated version of the Nvidia SoC will have its quad-core processor clocked at 1.7GHz (instead of 1.5GHz like in the international One X) and a GPU that's 25% faster.
According to the leaked shot, the HTC One X+ for T-Mobile USA will arrive in late September. Of course, this is based on hearsay, so take it with a pinch of salt. Also, whether or not the X+ will go international is an interesting question, but unfortunately one that can't be answered just yet.